In 1993, Professor Graetzel et al. at Ecole Polytechnique Fédérale de Lausanne, Switzerland, invented a dye-sensitized solar cell (See Patent Document 1 and Non-Patent Document 1 and 2).
The solar cell employs a sensitizer made from a ruthenium complex, and includes: a nanoporous film electrode of an oxide semiconductor having a wide bandgap, such as titanium oxide or zinc oxide in a form of nanoparticle; an iodine redox electrolyte solution; and a counter electrode. The solar cell has been attracting attention as one of next-generation solar cells, and actively researched and developed recently, because it has relatively high photoelectric conversion efficiency (solar energy conversion efficiency of 7% to 10%), and low-cost production potential.
The dye-sensitized solar cell invented by Graetzel et al. employs a complex as a photosensitizer, which complex containing ruthenium that is a precious metal. For large-scale electric power generation, the solar cell needs a lot of a ruthenium complex, so that it has been pointed out that the solar cell has a resource limitation problem.
With such a background, a dye-sensitized solar cell employing an organic dye as a photosensitizer, which organic dye contains no precious metals such as ruthenium, has been also researched and developed.
Examples of the organic dye that has been applied to the solar cell encompass: a phenylxanthene dye, a phthalocyanine dye, a cyanine dye, a merocyanine dye, a porphyrin dye, and an azo dye (See Non-Patent Documents 3 and 4). Particularly, a coumarin dye (See Non-Patent Document 5) is almost the same as the ruthenium complex in absorption wavelength range, and showed high photoelectric conversion efficiency. However, there has been a problem that the dye-sensitized solar cell employing the organic dye generally has a lower open circuit voltage and conversion efficiency than the solar cell employing the ruthenium complex.
For example, it has been known that the dye-sensitized solar cell employing the coumarin dye is lower than the solar cell employing the ruthenium complex in open circuit voltage because in a case of coumarin dye, the electrons spreading in titanium oxide have a shorter life time due to recombination than in a case of the ruthenium complex (See Non-Patent Document 6).
Development of the dye-sensitized solar cell, and use of the organic dye in the development are valuable in consideration of the future, and it has been imperative to research and develop the solar cell that has a lower open circuit voltage and higher conversion efficiency.    (Patent Document 1)    Japanese Patent No. 2664194 Specification    (Non-Patent Document 1)    Nature, 353, 737 (1991)    (Non-Patent Document 2)    J. Am. Chem. Soc., 115, 6382 (1993)    (Non-Patent Document 3)    Sol. Energy Mater. Sol. Cells, 64, 115 (2000)    (Non-Patent Document 4)    New J. Chem., 25, 200 (2001)    (Non-Patent Document 5)    J. Phys. Chem. B, 109, 15476 (2005)    (Non-Patent Document 6)    J. Phys. Chem. B, 109, 23776 (2005)